1. Field of the Invention
The present invention relates to surface acoustic wave filters and more particularly, to a surface acoustic wave filter having a ladder circuit construction that includes a balanced input/output.
2. Description of the Related Art
A plurality of conventional surface acoustic wave filters have been used as band-pass filters in mobile communication devices. Particularly, a variety of surface acoustic wave filters have been proposed which have a ladder circuit construction with low insertion loss and wide bandwidth.
FIG. 27 is a circuit diagram showing a basic construction of the surface acoustic wave filter which has the ladder circuit construction. A signal line which establishes connection between an input terminal 201 and an output terminal (not shown) defines a series arm. The series arm is connected to a series resonator 202. The series arm and the ground potential define a parallel arm therebetween. The parallel arm is connected to a parallel arm resonator 203. Although the filter circuit having a single stage is shown in FIG. 27, the surface acoustic wave filter usually has a multistage construction in which a plurality of series-arm resonators and parallel-arm resonators is provided.
The series-arm resonator and the parallel-arm resonator are constructed using one-port surface acoustic wave resonator. The one-port surface acoustic wave resonator is provided with an IDT electrode disposed in the center of the propagation direction of a surface acoustic wave and a reflector disposed on both sides of the IDT electrode in the propagation direction of the surface acoustic wave.
FIG. 28 shows one example of typical filtering characteristics of the surface acoustic wave filter having the above-described ladder circuit construction.
In EP0541284A2, a surface acoustic wave filter is disclosed having the ladder circuit construction in which an increase in the bandwidth as well as in the attenuation is achieved by adding an inductance component to the parallel arm resonator in series. Particularly, by constructing this inductance component via wire bonding, the size of a chip and its package are not increased.
That is, in an unbalanced surface acoustic wave filter in which a surface acoustic wave filter element and the package are electrically connected via a bonding wire, the inductance component of the bonding wire produces filtering characteristics thereof having a wider band as compared with filtering characteristics of a solely surface acoustic wave filter.
However, in the surface acoustic wave filter disclosed in EP0541284A2, the surface acoustic wave filter operates normally in an unbalanced-input-to-unbalanced-output circuit while not operating normally in a balanced-input-to-balanced-output circuit.
To solve this problem, a surface acoustic wave filter having the balanced ladder circuit construction corresponding to the balanced circuit is disclosed in U.S. Pat. No. 5,499,003. In FIG. 29, the circuit construction of the surface acoustic wave filter disclosed in this related art is shown.
Here, series arm resonators S1 to S4 and series arm resonators S5 to S8 are connected, respectively, to a first series arm defined by a signal line between a hot-side input terminal 211 and a hot-side output terminal 212 and a second series arm defined by a signal line between a ground-side input terminal 213 and a ground-side output terminal 214. In addition, a plurality of parallel arms are connected between the first and second series arms. Parallel arm resonators P1, P2, and P3 are connected to corresponding parallel arms.
As described above, by connecting parallel arms between the first and second series arms, a balanced surface acoustic wave filter is obtained.
Furthermore, a ladder surface acoustic wave filter corresponding to the unbalanced input/output is disclosed in Japanese Unexamined Patent Application Publication No. 6-69750. The ladder surface acoustic wave filter disclosed therein corresponds to the unbalanced input/output by optimizing the electrode capacitance of the series arm resonator and that of the parallel arm resonator.
In a balanced ladder surface acoustic wave filter disclosed in U.S. Pat. No. 5,499,003, an inductance component is not added to the parallel arm resonator. Accordingly, it is difficult to increase the bandwidth and the attenuation due to the above-described addition of the inductance component. In the surface acoustic wave filter described in this related art, the parallel arm resonator is limited to the surface acoustic wave resonator, and the band-pass width depends on a piezoelectric substrate. Consequently, the degree of freedom of band-pass width design is greatly reduced.
Furthermore, in the balanced ladder surface acoustic wave filter described in U.S. Pat. No. 5,499,003, electrical characteristics thereof can be measured only when the balanced circuit is connected thereto. Generally, network analyzers, which are used for evaluating electrical characteristics of the filter, are unbalanced circuits. Therefore, to measure electrical characteristics of the above surface acoustic wave filter using the network analyzer, a balance-to-unbalance transformer must be inserted therebetween.
One example of the above-described measuring circuit is shown in FIG. 30. In FIG. 30, reference numeral 221 represents a surface acoustic wave filter and reference numerals 222 and 223 represent balance-to-unbalance transformers. However, due to loss caused by the resistance and reflection included in these balance-to-unbalance transformers 222 and 223, highly accurate evaluation of filtering characteristics is very difficult to achieve. Furthermore, the operating frequency bands of the balance-to-unbalance transformers 222 and 223 are generally narrow and simultaneous evaluation of a wide frequency band is very difficult. In addition, variations in the balance-to-unbalance transformers 222 and 223 make it difficult to accurately and repeatedly evaluate the filter characteristics.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a surface acoustic wave filter having a ladder circuit construction which eliminates the problems with the foregoing related art, is capable of corresponding to the balanced input/output, has a wide frequency band and low loss, has a high degree of freedom of band width design, and is capable of accurately and easily having the filter characteristics thereof measured.
According to a first preferred embodiment of the present application, a surface acoustic wave filter includes a piezoelectric substrate and a plurality of surface acoustic wave resonators provided on the piezoelectric substrate wherein a ladder filter circuit including the plurality of surface acoustic wave resonators is constructed. The filter circuit includes input-side balanced signal terminals A1 and A2 connected to an external balanced circuit, output-side balanced signal terminals B1 and B2 connected to an external balanced circuit, at least one first series arm surface acoustic wave resonator connected in series between the input-side balanced signal terminal A1 and the output-side balanced signal terminal B1, at least one second series arm surface acoustic wave resonator connected in series between the input-side balanced signal terminal A2 and the output-side balanced signal terminal B2, and at least one impedance element connected between a first signal line connecting the input-side balanced signal terminal A1 to the output-side balanced signal terminal B1 and a second signal line connecting the input-side balanced signal terminal A2 to the output-side balanced signal terminal B2.
In a specific aspect of the surface acoustic wave filter according to the first preferred embodiment, at least one third surface acoustic wave resonator connected in series with the impedance element is provided.
In another specific aspect of the surface acoustic wave filter according to the first preferred embodiment, at least two of the third surface acoustic wave reonators are further provided.
In the surface acoustic wave filter according to the first preferred embodiment, as described below, the impedance element is added to the parallel arm resonator, wherein a wideband balanced or narrowband balanced surface acoustic filter is obtained. As described below, preferable characteristics which are not obtained by an unbalanced surface acoustic filter described in EP0541284A2 are obtained.
In a second preferred embodiment of the present application, a surface acoustic wave filter includes a piezoelectric substrate and a plurality of surface acoustic wave resonators provided on the piezoelectric substrate wherein a ladder filter circuit is constructed to include the plurality of surface acoustic wave resonators. The filter circuit includes input-side balanced signal terminals A1 and A2 connected to an external balanced circuit, output-side balanced signal terminals B1 and B2 connected to an external balanced circuit, at least one first series arm surface acoustic wave resonator connected in series between the input-side balanced signal terminal A1 and the output-side balanced signal terminal B1, at least one second series arm surface acoustic wave resonator connected in series between the input-side balanced signal terminal A2 and the output-side balanced signal terminal B2, and at least two third series arm surface acoustic wave resonators each led from a first signal line connecting the input-side balanced signal terminal A1 to the output-side balanced signal terminal B1 and a second signal line connecting the input-side balanced signal terminal A2 to the output-side balanced signal terminal B2, wherein a terminal of the third surface acoustic wave resonator which is not connected to the first signal line or the second signal line is grounded.
In the surface acoustic wave filter according to the second preferred embodiment, at least two third surface acoustic wave resonators are each led from the first signal line and the second signal line, and a terminal of the third surface acoustic wave resonator which is not connected to the first signal line or the second signal line is grounded. Therefore, as described below, wideband or narrow band filtering characteristics are obtained. In addition, without using a balance-to-unbalance transformer, filtering characteristics are very accurately determined.
In a specific aspect of the second preferred embodiment, an impedance element is inserted between the terminal of the third surface acoustic wave resonator which is not connected to the first signal line or the second signal line and a ground potential.
In a more specific aspect of the surface acoustic wave filter according to the second preferred embodiment, the ratio of the electrode capacitance of the third surface acoustic wave resonator led from the first signal line and the second signal line to the electrode capacitance of the third surface acoustic wave resonator led from the second signal line and the first signal line is approximately 1.4 or less. This enables the filtering characteristics to be more accurately measured. In addition, the balance is greatly improved.
In a more specific aspect of the surface acoustic wave filter according to the second preferred embodiment, at least one second impedance element is connected in series with the first series arm surface acoustic wave resonator between the input-side balanced signal terminal A1 and the output-side balanced signal terminal B1, and a third impedance element is connected in series with the second series arm surface acoustic wave resonator between the input-side balanced signal terminal A2 and the output-side balanced signal terminal B2.
Thus, by adding the impedance element to not only the series arm but also the parallel arm, greatly improved filtering characteristics are obtained. In particular, VSWR (reflection property) is greatly improved.
In the first and second preferred embodiments, the above impedance element is not particularly limited. An inductance element, a capacitance element, or a resonating element can be used as the impedance element.
When an inductance element is used as the impedance element, expansion of the bandwidth is achieved. When the capacitance element is used as the impedance element, narrowband filtering characteristics are obtained. When the resonating element is used as the impedance element, resonating characteristics having a capacitance ratio (the parameter C1/C0 which determines the distance between the resonating frequency and the anti-resonating frequency) that cannot be achieved by the surface acoustic wave resonator is obtained. As a result, the wideband or narrowband filtering characteristics are realized.
The inductance element may be defined by a bonding wire. Alternatively, a package for storing the piezoelectric substrate therein is further provided and a strip line that is connected to a signal line on the piezoelectric substrate is provided on the package, wherein the inductance element is defined by the strip line.
The above-described capacitance element may be defined by an electrode provided on the piezoelectric substrate. Alternatively, to construct the capacitance element on the piezoelectric substrate, an electrode for obtaining the capacitance may be discretely provided.
The above-described resonating element is not particularly limited, and can be constructed by combining inductance and capacitance elements.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.